The benefits of ultrasonics to examining soft tissue structures, particularly the abdominal region, brain, and eyes have long been known. In these applications, typically one or more acoustic contact transducers is used to generate and detect acoustic waves in the structure. These procedures are simplified, at least for examination of teeth, with the relatively large dimensions being examined, slower acoustic wave velocity (allowing lower frequency acoustic waves to be used for equivalent acoustic wavelengths), and readily available acoustic coupling material for the transducer to the soft tissue. (Soft tissue, unlike hard tooth enamel and dentin, is largely composed of water, making water a very efficient coupling material.)
Attempts to adapt conventional ultrasonic techniques to examination of internal structure of a tooth have met with little success. One major obstacle is identifying a suitable couplant for the transducer to the tooth for in-vivo measurements. Without proper coupling, transferring acoustic energy into the tooth is difficult. Early investigators attempted using water, as with soft tissue structures, but results were not convincing.
The coupling problem was overcome by replacing water with mercury. Although providing superior coupling efficiency, mercury is not suitable for clinical applications due to its toxicity.
Another solution to overcome the coupling difficulty was using a small aluminum buffer rod to transfer the acoustic energy from the contact transducer to the tooth. An estimated transmission efficiency of almost 87% was achieved using this technique, compared to only 5% using water. However, a significant limitation of this system was coupling the aluminum buffer rod with the tooth surface. To ensure proper coupling of the acoustic energy to the tooth, a flat spot had to be ground on the tooth surface, making this technique unsuitable for clinical applications. In addition, the relatively large contact area (3.2 mm diameter) limited the spatial resolution of the probe. For assessing anomalies in a tooth, such as poor bonding or voids between the restorative material and the dentin, a detection footprint smaller than the anomaly itself is required.
One method of increasing spatial resolution of a contact transducer is to use a spherical transducer that focuses abeam onto a sample (tooth) surface. This method forms the basis of the acoustic microscope, the acoustic equivalent of an optical microscope. This technique was used to study unblemished and demineralized enamel from extracted human teeth, using water as a couplant. The inspection depths were thus limited to approximately 0.5 to 1.5 mm.
More recently, the increased spatial resolution of the acoustic microscope was used to detect small caries lesions in sections of human enamel. However, as with previous work, special polishing of the tooth samples was required, making the technique ill-suited for clinical applications.
What is needed is a tooth structure assessment system achievable in-vivo operation that combines superior coupling efficiency, a small detection footprint size, and no special tooth surface preparation.